Fall Arrives in the Upper Midwest
It’s time to grab an apple cider donut and a pumpkin spice latte if you’re in the upper Midwest, because fall has arrived in the northern regions of the central United States. In the upper peninsula of Michigan, freeze warnings are out, and while other parts of the Midwest might not be quite so cold just yet, there is a definite nip in the air that wasn’t there a few weeks ago.
Of course, one of the most beloved characteristics of this seasonal transition is the change in the leaf colors. And what better way to track that then by satellite? Here’s an animation of the past two weeks of GOES-19 (GOES East) true color views courtesy of CSPP Geosphere. This animation shows a snapshot of the true color product at 1600 UTC (noon EDT, 11:00 AM CDT) so the solar illumination is roughly the same from one frame to the next. Watch how the tree colors in the upper midwest change from green to a rusty copper instead, at least when there aren’t any clouds in the way.
Something important to remember, however, is that the GOES true color view isn’t really a true color. Our eyes are sensitive to red, green, and blue. The GOES Advanced Baseline Imager (ABI) has a red channel and a blue channel, but it doesn’t have a green channel. Instead, this product uses a channel that is sensitive to vegetation and shades that as green in this RGB product. Basically, ABI can’t see green, but it knows what a lot of the green things look like in the near infrared, and so we pretend that when we see those things using a near IR channel, we can act as those things are actually green.
But other satellite imagers have a “true” true color product. VIIRS, for example, has a legitimate green channel. Let’s compare VIIRS to GOES ABI for the same scene and see how the two compare in their color rendition. Check out the slider below, in which GOES is on the left and VIIRS is on the right. Note how the GOES view just has an overall more greenish cast than the VIRS view does. For much of the year, the great plains are covered in green vegetation and so that green color is good enough. But as the vegetation transitions into the autumn season, its colors change in a way that the GOES true color RGB wasn’t designed for.
So let’s take a look at the VIIRS observations in closer detail. Here’s another slider, this time showing the difference in the VIIRS true color product between 23 September and 2 October 2026. The change between the two is astonishing. It’s no wonder why fall tourism is such an important part of year in this part of the United States.
Frequent readers of the CIMSS Satellite Blog know that satellites are good for more than just pretty pictures, however. We can actually get useful quantitative values from these observations. The EU Copernicus Browser enables users to interact with the Sentinel series of satellites, including plotting time series of observations. The Sentinel-2 satellite has 13 bands, including true red, green, and blue channels. Here, we’ve implemented a normalized red/green index: [(G – R) / (G + R)]. This looks at the difference between the red and green channels and divides that by the sum of the two so that the values are always limited to be between –1 and 1. During the summer months, the red reflectance is relatively small compared to the green reflectance, so the values will be positive. As the season transitions to fall, however, the green tends to go away and redder colors start to dominate. As a result, this normalized channel difference will trend to negative numbers as the seasons shift and the leaves change their color. That’s exactly what we see happening over the last week of Sentinel observations over the Keweenaw Peninsula in Michigan’s Upper Peninsula. Look at the steady downward trend starting around September 22.
The CIMSS Satellite Blog encourages you to get outside and enjoy one of the best times of the year.